The present invention relates to an output power control system, and more particularly, to an output power control system for controlling output power from a power amplifier in a radio-frequency transmitter.
Radio-frequency transmitters are used to transmit radio-frequency signals in a variety of applications. Typically the transmitters are designed to meet standards that specify output power levels and output signal rise and fall times. In many applications, output power control circuits or systems control the output power from power amplifiers employed in the radio-frequency transmitters. One type of output power control circuit includes a feedback loop that feeds back a portion of the output signal to an input of the power amplifier to control the output power. One standard that is used is the EIA Interim Standard, IS-19-B, 3.1.3.3, January 1988, USA, for the 800 MHz cellular subscriber units of the EIA system. For example, this standard specifies that the rise and fall times of the transmitted output power signal be less than two milliseconds.
A conventional output power control circuit 10 for controlling the output power of a power amplifier 12 is illustrated in FIG. 1. Output power control circuit 10 is a voltage-controlled output power control circuit which couples a percentage of the transmitted output power signal through a detector diode circuit. Power amplifier 12 has a voltage supply connection at 18 and a ground connection at 20. Power amplifier 12 transmits an input signal received at an input 14 and provides an output signal at an output 22. The output signal at output 22 is routed through a coupler 24 to provide an output signal at 26. A portion of the output power sent through coupler 24 is routed to an input 28 of a signal detector 30. Signal detector 30 responds to the peak level of the output signal at 26. Signal detector 30 includes a detecting diode and produces a detection signal at 34. A comparator 32 compares a reference voltage at 36 with the detection signal at 34. Comparator 32 provides an automatic power control voltage (Vapc) to an input 16 of power amplifier 12 to bring the detection signal at 34 in conformity with the reference voltage at 36.
One disadvantage of the conventional output power control approach employed by output power control circuit 10 are losses which reduce available output power and system efficiency. The couplers typically available for radio-frequency amplification systems are large, expensive, and have poor directivity characteristics. The poor directivity of the couplers results in poor output power accuracy, especially when the radio-frequency load impedance varies. Since the peak current loads of the power amplifiers are not monitored with this approach, the currents can be driven well above nominal levels under certain operating conditions, thereby further reducing system efficiency. For battery applications such as for mobile telephones, the result is a reduction in available talk time.
Another conventional approach used to control the output power of a power amplifier 42 is illustrated in FIG. 2. FIG. 2 illustrates a conventional current-controlled output power control circuit 40 which uses a sense resistor 54 in series with the voltage supply connection at 56 to develop a voltage proportional to the power amplifier 42 current. Power amplifier 42 has an input signal at 44 and an output signal at 52. Power amplifier 42 has a voltage supply connection at 48 which is coupled to one end of a sense resistor 54, and has a ground connection at 50. Sense resistor 54 is coupled at the other end to the voltage supply at 56. Capacitor 57 is coupled across sense resistor 54 between the voltage supply at 56 and the connection at 48 of power amplifier 42 to shunt instantaneous currents around sense resistor 54 so that sense resistor 54 conducts an average current.
The approach illustrated in FIG. 2 uses a feedback loop to convert a current through sense resistor 54 to a power control voltage at 46. A comparator 58 provides a voltage at 50 which corresponds to the current through sense resistor 54. Comparator 62 compares the voltage at 50 to a reference voltage at 64 and provides an automatic power control voltage (Vapc) at 46 to control the gain of power amplifier 42.
One disadvantage of this second approach are losses which reduce available output power and system efficiency. The sense resistors can be expensive and can be physically large. The sense resistors can also cause significant voltage drops which reduce the output power availability and efficiency of the power amplifier. This approach also has the disadvantage of having the total power amplifier current being fed back within the control loop. This can result in a loss of power control accuracy because the total current does not necessarily vary in direct proportion to the output power.
In view of the above, there is a need for an improved output power control system such as employed for controlling output power from a power amplifier in a radio-frequency transmitter.
One aspect of present invention provides an output power control system which includes an amplifier configured to supply a first current, a reference source configured to supply a second current which is proportional to and less than the first current, and a feedback converter responsive to the second current to control a gain of the amplifier.